1. Field of the Invention
The present invention relates to a heat transfer material for heating, and particularly to a heat transfer material for heating comprising a plurality of thin layers provided on a metal or nonmetal substrate. The invention also relates to a heating unit and a heating apparatus, both of which use the same material.
2. Prior Art
In evaporation and heating apparatuses, a reliable supply of heat energy is important. In an evaporation apparatus, for example, water vapor generated by burning kerosene, fuel oil, coal or the like is used as heat energy However, such apparatuses generate NO.sub.x, SO.sub.x, CO.sub.2 and the like and thus cause environmental pollution. Such apparatuses also have a defect in that a quantity of heat cannot be easily controlled because such apparatuses are complicated in construction and thus a complicated control system is needed therefore. There is thus the need for developing a clean and simple heating apparatus.
On the other hand, clean heat energy is provided by an electric heating method. For example, a fluid flowing in a metal pipe may be electrically heated by resistance heating using a sheathed type heater or the like. Although this method is simple, it has the disadvantage that it easily produces local heating. Namely, since good adhesion or contact of the sheathed heater to the metal pipe cannot be obtained, there is a problem that heat transfer is subject to high resistance. In addition, when a fluid flowing in a pipe is an electrically insulating substance such as fuel oil or the like, the metal pipe may be directly electrically charged so that the fluid is heated by a skin current effect. Although this method is highly efficient, it has the drawback that it cannot be applied to fluids of electrically conductive substances such as Na.sub.2 SO.sub.4 aqueous solutions and the like.
On the other hand, in recent years, surface treatment techniques have been significantly developed, and various materials can be easily formed into thin films on a substrate by coating, thermal sprayed coating, CVD or physical evaporation, sputtering, ion plating or the like. A film thickness can also be changed within the range of 0.1 .mu.m, to 3 mm according to the characteristics of the method used. A multilayer film can be produced by the use of the thin film techniques stated above.
A method for forming an electric resistance heating layer by the use of the thermal sprayed coating, for example, is shown in Japanese Utility Model Application Public Disclosure No. 120597/83, Japanese Patent Application Public Disclosure No. 94394/84 and Japanese Patent Application Public Disclosure No. 154478/84.
However, when the thin film is formed by the thermal sprayed coating, since the minimum size of the film thickness is determined by the size of spray particles, the film thickness is usually limited to 50 .mu.m or more. Thus, the method is limited to a specific application.